Nodal and paranodal structure during Wallerian degeneration in frog spinal nerve

Early myelin breakdown following sural nerve crush: a freeze-fracture study

In this study we describe the early changes of the myelin sheath following surgical nerve crush. We used the freeze-fracture technique to better evaluate myelin alterations during an early stage of Wallerian degeneration. Rat sural nerves were experimentally crushed and animals were sacrificed by transcardiac perfusion 30 h after surgery. Segments of the nerves were processed for routine transmission electron microscopy and freeze-fracture techniques. Our results show that 30 h after the lesion there was asynchrony in the pattern of Wallerian degeneration, with different nerve fibers exhibiting variable degrees of axon disruption. This was observed by both techniques. Careful examination of several replicas revealed early changes in myelin membranes represented by vacuolization and splitting of consecutive lamellae, rearrangement of intramembranous particles and disappearance of paranodal transverse bands associated or not with retraction of paranodal myelin terminal loops from the axolemma. These alterations are compatible with a direct injury to the myelin sheath following nerve crush. The results are discussed in terms of a similar mechanism underlying both axon and myelin breakdown.

Electrophysiologic changes accompanying Wallerian degeneration in frog sciatic nerve

The time course of the physiological changes accompanying Wallerian degeneration in the frog is markedly prolonged in comparison with that in mammals. Following transection of frog sciatic nerve, stimulation of the distal segment results in muscle contraction of normal amplitude through day 4, after which tension and EMG signals decline rapidly to levels that are undetectable by day 7. The compound action potential in the severed nerve continues for a much longer period of time, however, persisting as long as approximately 6 weeks. The amplitude remains at normal levels for nearly 3 weeks, after which it declines progressively, approaching zero at approximately 6 weeks. Conduction velocity remains normal in both alpha- and beta-fibers for approximately 3 weeks; thereafter, velocity in the alpha-fibers declines to approximately 71% of normal, and beta-fiber conduction is no longer detectable. Contraction could be elicited by direct stimulation of the muscle during the entire 6-week period of the study. The failure of nerve-mediated muscle activity in the face of persistent nerve conduction could be the result of either a defect in the most distal portions of the nerve fibers or a defect in neuromuscular transmission. These results suggest that EMG evaluation may be more sensitive than nerve conduction studies for clinical assessment of axonopathy at early stages.

Taxol-induced neuropathy after nerve crush: long-term effects on Schwann and endoneurial cells

The present investigation is a continuation of previous studies showing taxol-induced changes up to 4 weeks after a nerve crush. To evaluate the long-term cellular response to taxol, we have extended our morphological analysis of these changes in the taxol-treated nerve crush for up to 40 weeks after a single injection of taxol (PI). The results showed that Schwann cells exhibited a long-lasting and marked response when taxol was injected into the crushed peripheral nerve. During the first 2 months PI, taxol-induced giant axonal bulbs showed the formation of primitive nodes of Ranvier as a result of Schwann cell invaginations. The Schwann cell invaginations developed into nodes of Ranvier after 3-4 months PI together with the recovery of axonal bulbs. Ultrastructurally, cytoplasmic microtubule-related abnormalities were numerous up to 3 months PI and microtubules were seen to enclose degenerative myelin. Taxol-induced abnormalities in Schwann cells did not prevent their ability to produce myelin sheaths, although the accumulation of microtubules between myelin lamellae caused swellings of Schmidt-Lanterman incisures and paranodal myelin loops. Abnormal, extracellular collagen-like 5-nm-thin fibrils were noted closely associated with Schwann cells up to 10 weeks PI. Endoneurial cells, present as long rows without interconnections were noted in areas devoid of axonal sprouts up to 6-8 weeks PI. These cells showed marked cytoplasmic elongations and were covered by thickened basal lamina and contained several microtubule-related cytoplasmic structures, some of which have not been described previously. Taxol, when injected into crushed sciatic nerve induced a long-lasting response upon the Schwann cells with several ultrastructural abnormalities which correlate with changes in myelination and the development of nodes of Ranvier. These findings suggest that normal microtubule turnover is necessary for Schwann cells during nerve fiber regeneration.

Acid phosphatase activity at nodes of Ranvier in alpha-motor and dorsal root ganglion neurons of the cat

Acid phosphatase (AcPase) activity in feline alpha-motor and dorsal root ganglion (DRG) neurons was analysed histochemically by light and electron microscopy. The occurrence and distribution of the AcPase activity expressed within the axon differed depending on neuron type and distance from the cell body. Both in alpha-motor and DRG neurons, AcPase-positive bodies of various morphological categories were observed mainly at nodes of Ranvier, where they were more frequent distal than proximal to the nodal midlevel. In the peripherally located processes of both neuron types, most of the larger AcPase-positive bodies were associated with the paranodal axon-Schwann cell network. In the centrally located processes the AcPase-positive bodies were situated in the constricted axon segment and the adjacent paranodal axoplasm. Both in motor and DRG axons, AcPase-positive bodies were more frequent at the spinal root level than at a level central to the PNS-CNS borderline. The observations indicate that lysosomes (i.e. AcPase-positive bodies) constitute part of the intra-axonal system of organelles in normal, large, myelinated alpha-motor and DRG axons of the cat. Lysosome-mediated degradation of retrogradely transported endogenous and exogenous materials may be extensive in normal peripherally directed neuronal processes. The study also suggests a difference between PNS and CNS parts of the same axon with regard to the local turnover of lysosomal organelles.

Nodal and paranodal structural changes in mouse and rat optic nerve during Wallerian degeneration

Ultrastructural changes in nodal and paranodal regions of myelinated mouse and rat optic nerve fibers were followed between 4 h and 28 days during the course of Wallerian degeneration. In the mouse, axoplasmic changes, including accumulation of organelles and segregation of microtubules, were detectable 4 h after transection, and progressed to a maximum level on day 4, at which time many axons were markedly swollen. Dense axoplasm was seen as early as 16 h and was a common feature of degenerating axoplasm at later times. Paranodal changes, which first appeared as early as 16 h after injury, included detachment of terminal loops of myelin from the axolemma, disconnection of terminal loops from compact myelin lamellae and broadening of terminal loops, or separation of the loops from each other, resulting in paranodal elongation. In freeze-fracture replicas, the E-face of the axolemma showed the normal particle distribution as late as days 3-5. By day 8, however, the nodal particles were patchy and the overall nodal particle density was reduced to approximately half normal. Some normal-looking fibers were present at all stages examined, but their number had declined to about half the total population on day 5 and to less than 10% on day 11. In the rat, the overall sequence of events and time course were comparable to those in the mouse. Thus, the morphological changes found follow approximately the same sequence as that described previously in frog nerves, but progress more rapidly in the mouse and rat.

Patterns of change in endoneurial capillary permeability and vascular space during Wallerian degeneration

In degenerating frog sciatic nerves an in situ perfusion technique was used to measure the permeability coefficient-surface area product (PA) of endoneurial capillaries to [14C]sucrose, and the endoneurial vascular space (V). Both PA and V started to increase after the 3rd day of degeneration. The increase in PA peaked around the 14th day of degeneration and then declined to reach near normal levels at 6 weeks post-transection. V increased until about the end of the 3rd week of degeneration and then declined to near normal levels at 6 weeks after transection. The delayed increase in capillary permeability may reflect an adaptive reorganization of endoneurial capillary structure and function in response to altered conditions of the endoneurial microenvironment, and it is suggested that this permeability increase is induced by breakdown products of axons or chemical signals from Schwann cells enveloping transected axons. Fluid extravasation from these leaking capillaries is probably responsible for the endoneurial oedema observed in Wallerian degeneration. The recovery of endoneurial capillary tone to near-normal levels at 6 weeks after transection leads to the intriguing conclusion that healthy nerve fibers are not essential for the maintenance of normal endoneurial capillary permeability.